Consider an example precision part such as a multi-layer semiconductor or circuit boards with numerous die as shown in FIG. 1A. Such parts are typically manufactured by laminating together multiple layers, each containing metallic conductors. Each layer is fabricated by first coating the layer with resist and then optically projecting, at different locations or sections of the layer, an image containing one or multiple die and then etching the metal to form the desired metallic circuit conductors. This procedure is typically referred to as a project, step and repeat process. Due to current manufacturing limitations, the placement of the projected patterns is known to vary slightly between the same layer design manufactured at different times or even between layers in the same manufacturing lot. This is especially true for large circuit panels manufactured on thin flexible organic substrates, some of which are currently on the order of 500 millimeters×500 millimeters in size.
To facilitate the manufacture and optical inspection of all the die on the substrate, alignment marks are frequently placed in each individual die pattern. During the inspection process the substrate is typically scanned either by mechanically moving the substrate beneath an inspection camera or moving the camera over the substrate, or some combination thereof. Alignment marks, or unique conductor patterns within each die or die subsection, are located in the scanned images and used to align the scanned images to stored reference images of a good die or die subsection. Once aligned, the images are superimposed, and defects are reported in those locations where the stored reference and scanned camera images differ. As an example, FIG. 1B illustrates a reference image of a conductor, shown in black. FIG. 1C illustrates a scanned camera image of a conductor, shown in gray, with a hole 101. FIG. 1D illustrates a perfect superposition of the scanned camera image of FIG. 1C placed over the reference image of FIG. 1B. Hole 101 is detected as a defect where the superimposed images differ.